Update readthedocs config (home-assistant#65230)

Co-authored-by: Martin Hjelmare <[email protected]>

Author: scop

.readthedocs.yml

@@ -1,10 +1,14 @@
 # .readthedocs.yml
 
+version: 2
+
 build:
-  image: latest
+  os: ubuntu-20.04
+  tools:
+    python: "3.9"
 
 python:
-  version: 3.8
-  setup_py_install: true
-
-requirements_file: requirements_docs.txt
+  install:
+    - method: setuptools
+      path: .
+    - requirements: requirements_docs.txt